This invention relates to a system including a variable rate modulator for (1) varying the rate at which signals are modulated in accordance with variations in the rate of introduction of digital data to the system and (2) processing the modulated signals to provide output signals at a fixed frequency.
In recent years, a number of different applications have arisen in which digital signals representing data are processed and the processed signals are then converted to analog signals. For example, such applications have included the transmission of television signals through coaxial lines to homes. In such systems, the digital data is converted to analog signals and the analog signals are then transmitted through coaxial lines to homes of subscribers. Other applications are in microwave links to satellite communications.
In a number of the different applications involving the processing of digital data and the conversion of the processed digital data to analog signals, the digital data is provided at a variable frequency or rate and the analog signals are provided at a fixed frequency different from the variable frequency or rate. For example, the digital data may be provided in the range of approximately 10-40 megabits per second and the analog signals may be sampled at a fixed frequency of approximately 120 megahertz.
In the above example, the digital signals in the range of 10-40 megabits/second are converted to an intermediate frequency having a fixed value. For example, the digital signals in the range of 10-40 megabits may be converted to signals at an intermediate frequency of approximately 5 megahertz. The signals at the intermediate frequency are then used to modulate the signals at the fixed sampling frequency of approximately 120 megahertz.
As will be seen from the above discussion, a considerable range of frequencies (e.g. 10-40 megabits/second) has to be converted to a single fixed intermediate frequency (e.g. 5 megahertz). This is not easy. If the conversion is not accurate, the signals at the intermediate frequency jitter. When the signals illustratively provide television information, the jitter produces a significant deterioration in the quality of the television image.
This invention provides a system for, and a method of, converting digital data signals variable through a wide range of frequencies or rates into signals at a fixed intermediate frequency. This conversion occurs without any jitter in the signals at the fixed intermediate frequency. The signals at the fixed intermediate frequency then modulate signals at a fixed sampling frequency significantly higher than the fixed intermediate frequency. When the system of this invention is illustratively used to provide television images, the television images have a high resolution.
In one embodiment of the invention, clock signals and digital data signals at a variable frequency are introduced to an input of a FIFO (first-in, first-out) and are passed from the FIFO at a second (or intermediate) frequency controlled by a numerically controlled oscillator. To regulate the frequency of the signals from the numerically controlled oscillator, the phases of the clock signals at the variable frequency are compared in a phase detector with the phases of the signals from the numerically controlled oscillator to generate an error signal.
The error signals and the signals at a fixed sampling frequency higher than the intermediate frequency regulate the frequency of the signals from the numerically controlled oscillator and thus, the frequency of the digital data signals from the FIFO. The digital data signals from the FIFO are converted to a pair of signals at the second frequency. The pair of signals at the second frequency have individual ones of a plurality of analog levels dependent upon a code indicated by successive pairs of the digital data signals.
The signals at the second (or intermediate) frequency modulate a pair of trigonometrically related signals at the fixed sampling frequency. The modulated signals at the fixed frequency are combined and the combined signals are converted at the fixed sampling frequency to corresponding analog values by a digital-to-analog converter.